1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a method of fabricating metal particles with a pyrolyzed coating for battery anode applications.
2. Description of the Related Art
The rechargeable lithium ion battery (LIB) has triggered the portable electronic devices revolution due to its high power density, long cycling life, and environmental compatibility. The rechargeable LIB consists of a cathode (positive electrode) and an anode (negative electrode), separated by a Li+-ion permeable membrane. A solution or polymer containing lithium-ions is also used in the battery so that Li+-ions can “rock” back and forth between the positive and negative electrode freely. The positive materials are typically transition-metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and their derivatives. Lithium-ions can move in their interstitial space freely and reversibly. The negative electrode materials can use lithium-metal, alloys, and carbonaceous materials. During discharge, Li+-ions are extracted from the negative electrode and inserted into the positive electrode. In the meantime, electrons pass through an external circuit from the negative electrode to the positive electrode and generate electric power. During a charge, ions and electrons move along the reverse direction and go back to their original places.
Although LIBs have been successfully used, the conflict between lithium demand and its scarcity surges its cost, which hinders the further application of lithium-ion batteries on a large scale. Therefore, a low-cost rechargeable battery is urgently needed as an alternative to expensive LIBs. Under the circumstance, sodium-ion batteries are attracting attention because sodium has very similar properties to lithium, but a cheaper cost. Like lithium-ion batteries, sodium-ion batteries need Na+-host materials as their electrode. Much effort has been expended to directly duplicate the Li+-host structures, using Na+-host electrode materials for the sodium-ion batteries. For example, NaCoO2, NaMnO2, NaCrO2 and Na0.85Li0.17Ni0.21Mn0.64O2, all having a layered-structure similar to LiCoO2, have been developed for sodium-ion batteries. Similarly, Co3O4 with a Spinel structure, Na3V2(PO4)3 with a NASICON structure, and NaFePO4 with an Olivine structure have been employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F and Na1.5VOPO4F0.5, have also used as the positive electrode in sodium batteries.
Although sodium metal is a good choice for sodium-ion battery (SIB) anode, safety issues exist for commercial SIBs, such as flammability, dendrite growth during charge/discharge, and a low melting point. Therefore, efforts are being made to develop non-sodium anode materials for SIBs.
Currently, non-sodium metal anodes can be mainly put into two categories, carbonaceous materials and metals/metal chalcogenides (oxides and sulfides). Carbonaceous materials have three allotropes, which are diamond, graphite, and buckminsterfullerene [1]. In application to metal-ion batteries, only graphite and its disordered forms are practical materials as anode materials. Graphite typically has a layered structure into/from which lithium-ions can reversibly intercalate/deintercalate. But large sodium-ions and potassium-ions are hard to insert into the layered structure, which demonstrated a low capacity [2]. With certain treatments, carbonaceous materials become amorphous. Based on their crystalline structure, the amorphous carbon materials are classified as either “soft carbon” (graphitizable carbon) or “hard carbon” (non-graphitizable carbon). The amorphous carbon materials have demonstrated good performance as the anode material in SIBs. Carbon black, a kind of soft carbon, was reported as the anode materials in SIBs with sodium being reversibly inserted into its amorphous and non-porous structures [3]. Its reversible capacity was about 200 milliamp hours per gram (mAh/g), between 0-2 V vs. Na/Na+. The carbon black materials have an almost negligible porosity, so researchers believe that the large external surface areas may facilitate a reaction with sodium.
Hard carbon was also investigated in SIBs [4]. The sodium intercalation in hard carbon can be considered to occur in two steps. In a high voltage range, sodium-ions insert into the parallel graphene layers. In a low voltage range, sodium-ions intercalate into the pores of hard carbon. Although the behavior of a hard carbon anode in the low voltage range is favorable for SIBs, its capacity is relatively low.
The other category, metals/metal chalcogenides are a very promising SIB anode material. According to calculation [5], every tin and lead molecule can alloy 3.75 sodium atoms, corresponding to 847 mAh/g and 485 mAh/g for tin and lead, respectively. In addition, a SnSb alloy was reported as the anode material for SIBs [6]. The material showed a reversible capacity of 544 mAh/g during charge/discharge. In 50 cycles, its capacity retention was 80%. Aside from metals, Sun, et al., reported a Sb2O4 thin film as a SIB anode [7]. It exhibited a large reversible capacity of 896 mAh/g that originated from the alloying/dealloying and oxidation/reduction processes of antimony. Like Sb2O4, spinel materials of Co3O4 and Li4Ti5O12 showed a very similar behavior [8]. These materials in half cells with sodium counter electrodes had the discharge voltage of ˜0.5V and charge voltage of ˜1.0 V. The reversible capacity of Co3O4 was about 350 mAh/g, and that of Li4Ti5O12 was about 100 mAh/g. NiCo2O4 is another choice as a SIB anode [9]. Sodium-ions reacted with the material from 1.2V to 0V vs. Na/Na+, and were removed from it between 0.3V and 1.5 V vs. Na/Na+. Its reversible capacity was ˜200 mAh/g. In addition to oxides, Ni3S2 was also developed for SIBs anode [10]. It was discharged from ˜1.1 V to 0.3V and charged from 1V to 1.8V vs. Na/Na+. Its reversible capacity was around 250 mAh/g.
Basing on the aforementioned studies, it is known that tin (Sn), antimony (Sb), and lead (Pb) exhibit high capacities when used in the anodes of sodium-ion batteries. However, the alloying process causes large volume changes, which pulverizes the metal electrodes and degrades the battery performance. The same pulverization process occurs with potassium-ion batteries.
It would be advantageous to develop metal anodes, for use in batteries, whose volume remains stable during battery charge and discharge processes.    [1] Z. Ogumi, M. Inaba, Electrochemical lithium intercalation within carbonaceous materials: intercalation process, surface film formation, and lithium diffusion, Bull. Chem. Soc. Jpn., 71 (1998) 521-534.    [2] M. M. Doeff, Y. Ma, S. J. Visco, L. C. De Jonghe, Electrochemical insertion of sodium into carbon, Journal of the Electrochemical Society, 140 (1993) L169-L170.    [3] R. Alcántara, J. M. Jiménez-Mateos, P. Lavela, J. Tirado, Carbon black: a promising electrode material for sodium-ion batteries, Electrochem. Commun. 3 (2001) 639-642.    [4] X. Xia, J. R. Dahn, Study of the reactivity of Na/hard carbon with different solvents and electrolytes, Journal of the Electrochemical Society, 159 (2012) A515-A519.    [5] V. L. Chevrier, G. Ceder, Challenges for Na-ion negative electrodes, Journal of the Electrochemical Society, 158 (2011) A1011-A1014.    [6] L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Mie, J. Liu, High capacity, reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications, Chemical Communications, 48 (2012) 3321-3323.    [7] Q. Sun, Q.-Q. Ren, H. Li, Z.-W. Fu, High capacity Sb2O4 thin film electrodes for rechargeable sodium battery, Electrochem. Commun., 13 (2011) 1462-1464.    [8] Y. Kuroda, E. Kobayashi, S. Okada, J. Yamaki, Electrochemical properties of spinel-type oxide anodes in sodium-ion battery, 218th ECS meeting, abstract #389.    [9] R. Alcántara, M. Jaraba, P. Lavela, J. L. Tirado, NiCo2O4 spinel: first report on a transition metal oxide for the negative electrode of sodium-ion batteries, Chem. Mater., 14 (2002) 2847-2848.    [10] J.-S. Kim, G.-B. Cho, K.-W. Kim, J.-H. Ahn, G. Wang, H.-J. Ahn, Current Applied Physics, 11 (2011) S215-S218.